Several important mobility management issues need to be addressed in designing a communications system wherein the system is characterized by a dynamic network topology (e.g., mobile system access points or network nodes) and a relatively large number of mobile end users. Particularly, critical among them are issues such as: keeping track of mobile end users and access point locations in the system (i.e., location management); reaching and initiating communications with a mobile end user (i.e., mobile access); and continuity of service when a mobile end user moves between coverage areas associated with system access points (i.e., handoff management). Existing networks do provide solutions to these problems in their specific domains. As prior art, we discuss the solutions provided by Cellular Digital Packet Data (CDPD) networks, Cellular/Personal Communications Services (Cellular/PCS) systems, and Mobile Internet Protocol (mobile IP) networks.
CDPD networks consist of Mobile Data Intermediate Systems (MDISs), Mobile Data Base Stations (MDBSs), Mobile End Stations (MESs). An MDBS offering CDPD services helps a roaming MES register with the MDIS (called the serving MDIS) with which the MDBS is associated by acting as a conduit for the registration message. The serving MDIS informs the home MDIS of the MES of the latter's presence in its coverage area. When a host needs to send data to an MES, it does not have to be aware of the mobility aspect of the MES, it simply transmits data using the MES's IP address as the destination address. The IP packets are terminated at the home MDIS which encapsulates them in new data packets using the serving MDIS address as the destination address. The encapsulated data packets for the MES are forwarded to the serving MDIS of the MES. At the serving MDIS, packets are de-capsulated to reveal the MES's address. The serving MDIS sends the original data packets to the right channel where the MES is currently located. The MES receives the data packets. If the MES needs to reply, it directly sends data packets using the remote host's IP address as the destination address.
Intra-MDIS (i.e., between the two MDBSs served by the same MDIS) handoffs in CDPD are quick and simple because the data link between the MDIS and the mobile remains intact. However, Inter-MDIS handoffs could interrupt service since the data link will most likely be torn down when a mobile moves between MDISs. In this case, users have to reregister with the new serving MDIS. CDPD systems are further described in K. Budka et al., "Cellular Digital Packet Data Networks," Bell Labs Technical Journal, Vol. 2, No. 3 (Summer 1997); "Cellular Digital Packet Data Systems Specification: Release 1.1," CDPD Forum, Inc., Chicago (1995); and M. S. Taylor et al., "Internetwork Mobility: The CDPD Approach," Prentice Hall PTR, Upper Saddle River, N.J. (1996).
In Cellular/PCS systems, Home Location Registers (HLRs) and Visitor Location Registers (VLRs) are used to handle mobility management. HLRs and VLRs potentially can reside anywhere in the network. An HLR contains profile information about each of its mobile subscribers and the address of the current VLRs of its mobiles. Each Mobile Switching Center (MSC) has a VLR which tracks all mobiles currently receiving service in the serving MSC's coverage area. Whenever a mobile enters an area served by a new VLR and registers itself there, the latter informs the mobile's HLR of the change in the mobile's location. In addition, the VLR downloads the service profile of the roaming mobile as well as other information necessary for call termination at the mobile.
When a mobile terminated call is initiated, the first PSTN switch encountered by the calling party realizes that the called party number is a cellular/PCS number, thus it queries its HLR for the mobile's location. The HLR in turn contacts the current serving system and obtains a temporary local directory number (TLDN) from the current VLR. Using the TLDN, the first switch sets up the circuit to the serving MSC.
Typically, there are two types of handoffs in cellular/PCS networks: hard and soft handoffs. During a hard handoff, a mobile prepares a new, better quality link but switches to it only after it drops the air link between itself and its communicating node. During a soft handoff, a mobile can simultaneously have several air links (some of these can be dropped later when they are no longer needed). This phase can last as long as necessary to maintain good quality for the call. Soft handoffs are widely used by CDMA while hard handoffs are widely used by North American TDMA and GSM (also a TDMA system), as well as by AMPS (an FDMA system). Cellular/PCS systems are further described in "ISDN Based C Interface Access for PCS CDMA," Special Report SR-3797, Issue 1, Bellcore (December 1995).
While CDPD and Cellular/PCS networks were driven by the need to support wireless end users that may move when they are in a dormant mode as well as in an active (i.e., communicating) mode, mobile IP has a different motivation. It was driven by the need to support end users that would access the network from different points at different times. A mobile IP network consists of mobile nodes, home agents and foreign agents. A home agent is a router that authenticates a mobile node, tracks a mobile's location, and redirects data packets to the mobile's current location. A home agent maintains a table of all the mobile nodes that are homed to it with fields such as mobile's home address and mobile's care-of-address. A foreign agent assists the mobile node in informing its home agent of its current location, routes data traffic sent by the mobile, and sometimes provides the care-of-address and data packet de-capsulation for the mobile node. A foreign agent also maintains a list of visiting mobile nodes and their information.
Similar to CDPD, mobile IP also uses triangle routing. A mobile's home agent receives packets destined to the home address of the mobile and tunnels them to the care-of-address of the mobile node by encapsulating the original IP packets in new IP packets with the destination address set to the mobile's care-of-address. At the care-of address, the original packets are extracted from the tunnel and then delivered to the mobile node. In the reverse direction, packets are sent by the mobile directly to the remote host without tunneling
Mobile IP with route optimization and smooth handoff is still in the Internet Engineering Task Force (IETF) draft stage, see C. E. Perkins et al., "Route Optimization in Mobile IP," draft-ietf-mobileip-optim-07.txt (Nov. 20, 1997). A mobile in handoff asks its new foreign agent to send a binding update to its previous foreign agent. During handoff, the previous foreign agent tunnels the transient packets to the new care-of-address. In case the previous foreign agent has no fresh binding, it can forward the transient packets to the home agent through a special tunnel which prevents routing loops from forming between the home agent and the previous foreign agent. Mobile IP is further described in C. E. Perkins, "Mobile IP," IEEE Communications Magazine, pp. 84-99 (May 1997); and J. D. Solomon, "Mobile IP: The Internet Unplugged," Prentice Hall (1998). Mobility issues are further described in "I. F. Akyildiz et al., "Mobility Management in Current and Future Communications Networks," IEEE Network, pp. 39-49 (July/August 1998). Also, mobile IP point-to-point communication is described in U.S. Ser. No. 09/150,403, filed on Sep. 9, 1998, and U.S. Ser. No. 09/074,582, filed on May 8, 1998, both entitled "A Mobile Point-to-Point Protocol," the disclosures of which are incorporated herein by reference.
However, there are several drawbacks to the individual approaches described above. For instance, as is known, mobile IP networks are typically hampered by route inefficiency problems. On the other hand, as is also known, cellular/PCS networks require significant signaling overhead during call setups and handoffs. As such, there is a need for a network architecture and mobility management techniques that yield an efficient, scaleable, and flexible communications system capable of handling various applications including multimedia applications.